Functional analysis of the Wnt-Irx regulatory axis in the chondrocyte-osteoblast lineage and metabolic homeostasis

Abstract

It has been established that chondrocytes and osteoblasts comprise a lineage continuum by which hypertrophic chondrocytes (HCs) become osteoblasts and osteocytes, contributing to bone formation. However, the gene regulatory network that controls the chondrocyte to osteoblast transformation and cell fate decision is poorly understood. The possibility that a small proportion of HCs can also become adipocytes has been suggested, but the underlying mechanism has not been established. Here, I show that β‑catenin regulates gene expression of Irx3/5 in developing trabecular bone to determine the cell fate of HC descendants. The loss of β‑catenin and the downstream target IRX3/5 in the HC lineage results in significantly elevated numbers of adipocytes, contributing to an increased amount of bone marrow adipose tissue (BMAT), which becomes apparent in early neonates (P10). I found that the reduction in bone and increase in BMAT are mainly caused by an increase in the number of HC-derived bone marrow adipocytes and a decrease in the number of HC-derived osteoblasts, implicating Wnt signalling and IRX3/5 as key regulators of osteogenesis versus adipogenesis in trabecular bone formation. In Ctnnb1 CKO mice (conditional knockout of β‑catenin by Col10a1Cre), bone marrow adiposity is elevated with no genetic manipulation in peripheral adipose tissue, representing a novel mouse model for studying the function of bone marrow adipose tissue in global metabolism. Ctnnb1 CKO mice demonstrate significant metabolic changes, including a lower body fat composition and improved glucose tolerance and insulin sensitivity under a normal chow diet, which are likely consequences of an increased metabolic rate and increased thermogenesis/lipolysis activity. Furthermore, Ctnnb1 CKO mice are resistant to high-fat diet-induced obesity. The metabolic improvements in Ctnnb1 CKO mice suggest that bone marrow adipose tissue may be a novel endocrine organ involved in maintaining metabolic health and preventing the metabolic deficiency induced by unhealthy food intake. Interestingly, tdTomato expression driven by Col10a1Cre suggests that Cre recombinase may be activated in not only hypertrophic chondrocytes in the bone system but also various types of neurons in the olfactory system. The metabolic phenotypes of Ctnnb1 CKO mice have some degrees of similarity with mice with smell sensing deficiency under a high-fat diet. These findings raise the possibility that the olfactory system may also be related to the metabolic improvements in Ctnnb1 CKO mice, although the underlying mechanism requires further validation. In conclusion, my findings demonstrate that a Wnt-Irx regulatory axis controls HC lineage fate and whole-body metabolic homeostasis and extend the understanding of the relationship among trabecular bone mass, marrow adipose tissue and metabolic control with important implications for metabolic disorders, such as diabetes and obesity, and congenital and aged-related skeletal disorders, such as chondrodysplasia and osteoporosis. Bone marrow adipose tissue can be an emerging key regulator of global metabolism. In the future, the molecular basis underlying the control of metabolic homeostasis by marrow adipose tissue should be further explored, and the potential interaction between marrow adipose tissue and the nervous system requires further investigation.